Since acrylic pressure-sensitive adhesives are excellent in weather resistance, deterioration resistance, and adhesiveness, they are used for various applications, for example, for pressure-sensitive adhesive labels, pressure-sensitive adhesive sheets, and pressure-sensitive adhesive tapes, and under present conditions they are mainly of the solvent type or the emulsion type.
On the other hand, as matters now stand, hot-melt pressure-sensitive adhesives are used in the form of a composition whose base polymer is a styrene/isoprene/styrene block copolymer, and such adhesives are poor in weather resistance and deterioration resistance.
Along with the heightened demand for solventless pressure-sensitive adhesives, attempts to obtain acrylic pressure-sensitive adhesives of a hot-melt type have been made.
Acrylic, hot-melt, pressure-sensitive adhesives described, for example, in Japanese Patent Application (OPI) Nos. 75975/1984, 125774/1983, 161484/1981, and 315409/1989 are generally unsatisfactory in cohesive force of the pressure-sensitive adhesives or workability in hot-melt coating.
Although, as a means of lessening the above drawbacks, a moisture-curing hot-melt adhesive composition is suggested in Japanese Patent Application (OPI) No. 259984/1991, since the composition comprises an alkyl (meth)acrylate, a macromer (macromolecular monomer), a (meth)acrylate that has a silyl group, and an isocyanate compound, the workability in hot-melt coating before moisture-curing is not satisfactory, and in particular, the operations are restricted severely by the toxic gas generated from the isocyanate compound when the composition is melted at a high temperature.
Further, the moisture-curing adhesive composition described in Japanese Patent Application (OPI) No. 139584/1991, which comprises an alkyl (meth)acrylate, a polymerizable polymer, and an unsaturated isocyanate, suffers from the drawback that the change with the passage of time in melt viscosity is large. Although this moisture-curing adhesive composition contains a styrene tackifier resin, in order to lower the melt viscosity, the tackiness is poor because the tackifier resin and the alkyl (meth)acrylate part are inappropriately compatible. In order to lessen the drawback of the large change with the passage of time in melt viscosity, a moisture-curing hot-melt adhesive composition comprising an alkyl (meth)acrylate, a polymerizable polymer, an unsaturated isocyanate, an organotin compound, and an organophosphorus compound is disclosed in Japanese Patent Application (OPI) No. 31482/1992, but making the change in melt viscosity small by adding an organophosphorus compound results in the drawback that the moisture-curing speed becomes slow. Further, if graft copolymers described in Japanese Patent Application Nos. 139584/1991 and 31482/1992 are used alone, the initial adhesion before moisture-curing and the initial cohesive force are insufficient, even if the melt viscosity is brought to a suitable melt viscosity (100,000 cps or less) at a relatively low temperature (about 120.degree. C.), in order to attain excellent workability in coating.
In general, complete curing of moisture-curing hot-melt adhesive compositions must take at least 3 to 7 days, which causes problems with productivity and the like.
On the other hand, actinic-radiation-curing, hot-melt, pressure-sensitive adhesives described in Japanese Patent Application (OPI) Nos. 276879/1990, 119082/1991, and 220275/1991 are suggested, but they are accompanied by problems that the melting temperature is as high as 150.degree. C., the melt viscosity increases with time, and compounds having molecular weights less than 1,000 volatilize.
Although Japanese Patent Application (OPI) No. 1109873/1982 suggests an ionizing-radiation-curing adhesive composition comprising an acrylic copolymer having a molecular weight of 1,000 to 50,000, to which side chains having an olefinically unsaturated bond are introduced, the cohesive force and adhesive force of the obtained adhesive are not satisfactory, particularly in the adhesive thickness of more than 40 .mu.m.
Thus, as matters now stand, there are no acrylic, hot-melt, pressure-sensitive adhesive compositions that satisfactorily meet the demand of the market; and hot-melt pressure-sensitive adhesive compositions wherein the change in melt viscosity after the lapse of time before curing is small (melt viscosity stability is high), excellent coating workability in hot-melt is exhibited, and the adhesive force and the heat-resistant cohesive force after curing are excellent, are strongly desired.